1. Field of the Invention
This invention relates in general to a method and apparatus for optimizing the recovery time for a magnetic head, and more particularly, to a method and apparatus for improving baseline recovery of an magneto-resistive (MR) head using a programmable AC coupling pole.
2. Description of Related Art
A magneto-resistive head has been conventionally used for reproducing data from a magnetic recording medium. The magneto-resistive element makes use of its physical property wherein the electrical resistance of the magneto-resistive element is changed in response to a sensed intensity of a magnetic field. Thus, the magneto-resistive head may obtain the intensity of the magnetic field from a magnetic recording medium by detecting the change of the electrical resistance.
As mentioned above, a magneto-resistive (MR) transducing head exhibits a change in resistance when in the presence of a changing magnetic field. This resistance change is transformed into a voltage signal by passing a constant current through the MR element. The value of DC voltage, for a given head, is the product of the constant bias current and the total resistance between the head lead terminals.
However, the prior art reproduction of the recorded data with use of the MR element has the disadvantage that the reproduced waveform by the MR element involves nonlinear distortion as the change of the resistance of the MR element to the flowing magnet device is properly non-linear as described above. For example, thermal asperities can locally increase the strip temperature by more than 100 degrees centigrade which may cause a resistance change and consequently a voltage change. Further, thermal fluctuations may produce a resistance change in the elements by passing bias current through them. Accordingly, when a thermal asperity is encountered the operating point of the MR element gets shifted and the data that is being modulated by the head will be unreadable without sufficient error correction processes.
Electronically, this change in the operating point of the MR element causes a baseline shift in the read-back signal. The circuits which are used to amplify the MR head signal will attempt to recover from the baseline shift in order to provide a readable signal. While the circuits are recovering, the data in many cases is not readable and must be recovered by error correction codes or a hard error will occur. The most likely cause of a hard error during a baseline shift is from the amplifiers saturating or from the distortion when the amplifiers are near saturation. To reduce the recovery time from an operating point disturbance and reduce the time the read back signal is saturated in the amplifier, faster recovery time constants can be chosen in the amplifier electronics. The pole that sets the recovery time constant is known as the AC coupling frequency pole of the amplifier or the high pass frequency pole of the amplifier. In the MR pre-amplifier, it can be known as the baseline restore bandwidth.
The problem with setting the AC coupling pole too high is that with the higher pole, data reading problems can occur from too much phase shift and amplitude reduction at the low frequency data patterns.
It can be seen that there is a need for an adjustable AC coupling pole for optimizing the recovery time of a magnetic head.